The present invention relates to an aluminum alloy powder suited particularly as a pigment for design coating materials, and also relates to a coating material containing this alloy powder.
Aluminum (Al) powder has been hitherto used as a pigment for coating materials for the purpose of, e.g., preventing corrosion, providing decorative coatings, or reflecting heat radiations. Such coating materials are used in the coating of automobiles, tanks, steel frames or skeletons, roofs, etc. This Al powder is comprised of leaf-shaped particles. When the powder is mixed in a resin material and the mixture is coated by brushing or spraying, the Al powder particles are laid overlapping in parallel with the coating layer owing to the surface tension produced in the curing of the resin (i.e., a leafing phenomenon) to form a continuous film comprised of the powder, thus making it possible to protect materials from the air to impart good corrosion resistance, or utilize the reflecting properties of Al powder to impart decorative appearance and weatherability.
Conventional Al powders are prepared by making granular powder from an Al melt by air atomizing or inert gas atomizing, pulverizing the resulting powder in a ball mill together with stearic acid or oleic acid to make it into a fine powder and at the same time stretch it using a shear force to have a flat particle shape. The powder thus obtained, however, is known to show poor reflecting properties when used in a coating material, because it tends to have rough particle surfaces and hence tends to have an irregular shape in its particle peripheries.
Meanwhile, in recent years, regulations for protecting the environment of the globe have been more tightened, and coating materials also have a tendency that those making use of organic solvents are replaced by water-based coating materials. This brings about the problem that, when water is used as a solvent, Al powder undergoes corrosion of particle surfaces to cause a deterioration of the reflecting properties. For this reason, Al powders whose particle surfaces are coated with phenol resin have been put into use, but still can not avoid causing a lowering of the reflecting properties because of the coating with resin.
The present applicants have already proposed a method of preparing an aluminum alloy powder having an amorphous phase or an amorphous and finely crystalline mixed phase, comprising forcing a melt of an aluminum alloy to flow out from a nozzle, jetting a gas to the melt to form droplets of the melt, and bringing the droplets into collision, before they solidify, against the surface of a rotating cooling member having the shape of a cone or horn and provided in the direction of the flow of the droplets, followed by rapid cooling to effect solidification, as disclosed in Japanese Laid-open Patent Application No. 1-319606, corresponding to U.S. Pat. No. 4,891,068. This publication discloses an aluminum alloy powder having the composition represented by the general formula: Al.sub.a M.sub.b X.sub.c, wherein M represents one or more of metallic element(s) selected from the group consisting of V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Ti, Mo, W, Ca, Li, Mg and Si; X represents one or more element(s) selected from the group consisting of Y, La, Ce, Sm, Nd, Hf, Nb, Ta and Mm (Misch metal); and a, b and c are 50.ltoreq.a.ltoreq.95, 0.5.ltoreq.b.ltoreq.35 and 0.5.ltoreq.c.ltoreq.25 in atom %, respectively), and comprising a particle having a thickness of 0.1 to 5 .mu.m, a minor axis of at least 5 .mu.m and a major axis not more than 500 .mu.m, an aspect ratio (which is the ratio of the major axis to the thickness) of not less than 5.
The above aluminum alloy powder comprises an amorphous phase or an amorphous and finely crystalline mixed phase, and hence it has the advantages that a superior corrosion resistance can be achieved, and less deterioration of reflecting properties may occur even when it is added to water-based coating materials.
This aluminum alloy powder comprising an amorphous phase or an amorphous and finely crystalline mixed phase, however, has relatively so high a specific gravity of the powder that it has the problem such that it can not be well dispersed when added as a pigment to a coating material to give a poor dispersibility when sprayed, and can not attain a sufficient hiding power of the material when mixed in a small quantity.